This invention relates to a protection system for preventing damage to circuit and/or mechanical components, which are driven or powered by rectified a-c power line voltage, that may otherwise be caused by undesired line voltage interruptions.
A controlled rectifier bridge, usually followed by a low-pass filter, is customarily employed to produce, from a-c power line voltage, an adjustable amplitude d-c bus voltage for powering a load in accordance with the level of the d-c voltage. In one well-known arrangement, the conduction angle of the SCR's (silicon controlled rectifiers) in a phase-controlled SCR rectifier bridge, to which is applied a-c line voltage, is controlled to develop a d-c output bus voltage of a desired magnitude for application over a d-c bus to a load, such as an inverter-motor system, the inverter converting the d-c bus voltage to a-c voltage for driving the motor. The rectifier bridge is automatically controlled to maintain the d-c bus voltage at the desired level. This is achieved by a feedback loop which compares a feedback signal, representing the actual amplitude of the d-c bus voltage, with a set point or command signal representing the desired magnitude for the d-c bus voltage. The error signal, produced from the comparison, controls the conduction angle of the SCR's in the rectifier bridge so as to hold the d-c bus voltage at the desired amplitude despite load variations, namely changes in the current drawn by the motor. In this way if the mechanical load on the motor changes, the motor current (and thus the d-c bus current) will likewise change and this tends to vary the d-c bus voltage. However, the feedback loop automatically compensates and varies the error signal in order to change the conduction angle of the SCR's as necessary to stabilize the d-c bus voltage at the desired set point level.
Unfortunately, undesired power interruptions or outages present a problem when a rectifier bridge, which rectifies applied a-c power line voltage, is automatically controlled to maintain its output d-c voltage constant in the presence of load current changes. When a power interruption occurs the load continues to draw current from the d-c power supply (namely, the rectifier bridge and an associated filter, if used) even though the bridge no longer receives input a-c power. This load current will continue in a decaying fashion for a time determined by the energy storage capacity of the system. For example, if the controlled rectifier bridge is followed by a shunt-connected filter capacitor, decreasing current will be drawn from that capacitor. Since there is no power available from the a-c power line to maintain the d-c bus voltage, that voltage will collapse or drop. This appears as an increased load to the feedback loop and it tries to compensate by increasing the error signal which in turn increases the conduction angle of the SCR's in the rectifier bridge. The longer the line voltage interruption, the greater will be the conduction angle in an unsuccessful attempt to have the SCR's deliver more power from the a-c power line. When power does return, the SCR's are being commanded to conduct at a large conduction angle (almost if not wide open), resulting in a sudden increase in the d-c bus voltage and very high d-c bus current. This rapid change could damage, or even destroy, the SCR's in the rectifier bridge and/or circuit components in the electrical load, such as switching devices when the load includes an inverter. A motor driven by the inverter could also be damaged by the sudden changes in bus voltage and current. Moreover, the motor will rapidly speed up and mechanical components driven by or connected to the motor could break or fail.
This problem has now been overcome by the present invention. The disclosed protection system renders the controlled rectifier bridge immune to the deleterious effects of a-c line voltage interruptions so that no sudden changes will occur in the d-c bus voltage and bus current when power resumes, thereby preventing damage to electrical and mechanical elements.